Artificial lighting systems of the type specified in the preamble are widely used, particularly in buildings. In order to provide an adequate lighting level in the environment controlled, the controller is operative to turn on the light source when the sensor detects that the daylight level has dropped below a pre-determined threshold, or in the opposite case, to turn off or dim the artificial lighting when the daylight level exceeds a predetermined threshold.
Apart from the obvious ergonomic aspects involved in adequate lighting of an environment, the lighting level also affects the human physiology. It is well known that the human physiological system is regulated by a mechanism that is commonly referred to as a biological clock. A plurality of physiological functions show a periodicity with a cycle of 24 hours. This repetitive behaviour is also called a circadian rhythm. It also is well known that lighting can dramatically affect this circadian rhythm. The mood and behaviour of an individual thus turn out to show a dependence on environmental lighting. Lighting optimized with regard to a person's preferences is found to contribute in a positive way to his or her sense of well-being and, hence, to his or her performance or behaviour. See, for example, U.S. Pat. No. 5,163,426.
"Interior Lighting", De Boer en Fischer, Second Edition, Philips Technical Library, Kluwer Technische Boeken, Deventer - Antwerpen, 1981, pp. 60-63 suggests that the level of permanent artificial lighting, which is necessary to give an acceptable brightness balance between the levels of artificial lighting and of daylight, is directly proportional to the level of exterior daylight. If the brightness balance is neglected, an object seen against a bright background is thrown into silhouette which is considered undesirable.
European patent application EP-A 0 410 484 discloses an artificial lighting system wherein, within a certain range of daylight levels, the level of artificial lighting is made substantially proportional to the level of the daylight entering into the room. Varying the artificial lighting in this manner is done for the same reason as above, namely, to avoid silhouettes.